摘要:

AbstractApplications of fMRI in functional brain imaging are mainly confined to single subject designs, prohibiting the assessment of subject or group by condition interactions (i.e., differential activations) or areas of conjoint activation. In this paper a framework for fMRI group designs, using statistical parametric mapping, is introduced. It is generally believed that intersubject averaging, which requires spatial normalization and smoothing, will decrease the effective spatial resolution of fMRI or its sensitivity. A subcortical activation of the lateral geniculate nucleus (LGN) was therefore chosen to demonstrate the feasibility and power of intersubject averaging in the context of fMRI. Seven volunteers were studied, while looking at a blank screen or radially moving dots. LGN activation was demonstrated in all single subject analyses and in the group analysis.

ISSN:0740-3194    

DOI:10.1002/mrm.1910380502

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractThe feasibility of brain MRI with laser‐polarized129Xe in a small animal model is demonstrated. Naturally abundant129Xe is polarized and introduced into the lungs of Sprague‐Dawley rats. Polarized xenon gas dissolves in the blood and is transported to the brain where it accumulates in brain tissue. Spectroscopic studies reveal a single, dominant, tissue‐phase NMR resonance in the head at 194.5 ppm relative to the gas phase resonance. Images of129Xe in the rat head were obtained with 98‐μl voxels by 2D chemical shift imaging and show that xenon is localized to the brain. This work establishes that nuclear polarization produced in the gas phases survives transport to the brain where it may be imaged. Increases in polarization and delivered volume of129Xe will allow clinical measurements of regional cerebral bl

ISSN:0740-3194    

DOI:10.1002/mrm.1910380503

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractA protocol is described for obtaining functional magnetic resonance images in anesthetized cat brain based on the blood oxygenation level dependent (BOLD) contrast mechanism. A visual stimulus was used, which consisted of a high‐contrast drifting grating, whose speed and spatial frequency was optimized for cat area 18 (V2). Experiments were conducted at 4.7 Tesla using a gradient echo EPI sequence with a 29‐ms echo time, yielding signal changes of between 0.7% and 2% in area

ISSN:0740-3194    

DOI:10.1002/mrm.1910380504

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractLactate NMR visibility was investigated in excised rat muscle at 3 T by comparing the concentration determinedin situby double quantum (DQ) proton spectroscopy (150 ms effective echo time) to the concentration measuredin vitrofrom perchloric acid extracts of the same muscle samples. After 1‐2 h of ischemia, lactate NMR visibility was 32 ± 3% (±SE,n= 9), and was only 21 ± 1% (n= 6) after 10–12 h. Muscle lactateT2was 140 ± 11 ms and 184 ± 6 ms, respectively. All potential mechanisms of DQ lactate signal attenuation (B0, andB1inhomogeneity, DQ transverse relaxation, diffusion) were examined, and accounted for when necessary. A significant increase in lactate NMR visibility was demonstrated using a shorter effective echo time (79 ms) DQ editing sequence. These results are interpreted as reflecting muscle lactate compartmentation between a longT2pool predominantly detected by DQ spectroscopy, and a shortT2pool virtually invisible with longer echo time NMR te

ISSN:0740-3194    

DOI:10.1002/mrm.1910380505

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractA series of linear copolymers of DTPA‐class Gd3+conjugates, linked by α,ω‐alkyldiamides with a varying number (n) of methylenes separating the amide function, were synthesized. Surprisingly, their relaxivities at all fields increased with increasingn. At MRI fields and 35°C, the relaxivities of then= 10 andn= 12 polymers were unexpectedly high, similar to those of rigid dendrimer‐based Gd3+chelates. The magnetic field dependence of solvent proton 1/T1was measured for aqueous urea‐free and urea‐containing polymer solutions. The results for urea‐free solutions imply an increase of rigidity (required for high relaxivities) with increasingn, arising from hydrophobic interactions of the methylene groups with solvent. This hypothesis is supported by a large decrease in the relaxivities upon addition of urea, which is known to weaken hydrophobic interactions. The relaxivities are also independent of polymer concentration, indicating that the hydrophobic interactions are

ISSN:0740-3194    

DOI:10.1002/mrm.1910380506

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractRapid quantification of tissue metabolitesin vivoby MRS or MRI can be achieved using dual‐frequency RF coils with identical B1field distributions at the observation frequencies of the metabolites and tissue water protons. Tissue sodium is used as an example for optimizing the dual‐frequency, dual‐quadrature RF coils for such measurements in humans. In the setting of sodium imaging, the challenge of dual‐quadrature birdcage configurations is to decouple the sodium and proton channels because the fourth harmonic of the sodium frequency is very close to the proton frequency. A generalizable method for effectively decoupling these two RF frequencies is presented in this paper. The method is demonstrated with the design of an EPI compatible, dual‐quadrature, double‐tuned,23Na/1H birdcage coil. The performance of the RF probe is reported at 1.5 Tesla in terms of signal‐to‐noise ratio, B1homogeneity an

ISSN:0740-3194    

DOI:10.1002/mrm.1910380507

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractThis paper presents the theory of the quadrature birdcage coil wound on an elliptic cylindrical former. A conformal transformation of the ellipse to a circular geometry is used to derive the optimal sampling of the continuous surface current distribution to produce uniform magnetic fields within an elliptic cylinder. The analysis is rigorous for ellipses of any aspect ratio and shows how to produce quadrature operation of the elliptic birdcage with a conventional hybrid combiner. Insight gained from the transformation is also used to analyze field homogeneity, find the optimal RF shield shape, and specify component values to produce the correct current distribution in practice. Measurements and images from a 16‐leg elliptic birdcage coil at both low and high frequencies show good quadrature performance, homogeneity, and sensitivit

ISSN:0740-3194    

DOI:10.1002/mrm.1910380508

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractThe point spread function is a fundamental property of magnetic resonance imaging methods that affects image quality and spatial resolution. The point spread function is difficult to measure precisely in magnetic resonance even with the use of carefully designed phantoms, and it is difficult to calculate this function for complex sequences such as echo‐planar imaging. This report describes a method that measures the point spread function with high spatial resolution at each pixel in samples of uniform intensity distribution. This method uses additional phase encoding gradients before the echo‐planar acquisition that are constant in length but vary in amplitude. The additional gradients are applied to image the contents within each individual voxel. This method has been used to measure the point spread function for echo‐planar imaging to demonstrate the effects of limitedk‐space sampling and transverse relaxation, as well as the effects of object motion. By considering the displacement of the point spread function, local distortions due to susceptibility and chemical shift effects have been quantified and corrected. The method allows rapid assessment of the point spread function in echo‐planar imaging,in vivo, and may also be applied to other rapid imaging sequences that can be modified to include these additional phase encoding

ISSN:0740-3194    

DOI:10.1002/mrm.1910380509

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractAn interleaved echo‐planar imaging (EPI) technique is presented for the rapid acquisition of isotropic diffusion‐weighted images of stroke patients. Sixteen isotropic diffusion‐weighted images at threebvalues are acquired in less than 3 min. A spiral navigator echo is used to measure the constant and linear phase shifts across the head inboththexandydirections which result from motion during the isotropic diffusion‐ sensitizing gradients. The measuredk‐space errors are corrected during a gridding reconstruction. The gridding kernel has a constant width inkxand a variable width inkywhich eliminates variable data‐density ghosts. The resulting isotropic diffusion‐weighted images have excellent lesion‐to‐normal brain contrast, very good spatial resolution, and little sensitivity to susceptibility effects in the base of the brain. Examples of diffusion‐weighted images and ADC maps from several stroke

ISSN:0740-3194    

DOI:10.1002/mrm.1910380510

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractThe stimulation of axons under MRI magnetic fields is analyzed solving the cable equation along the axons in the presence of magnetic fieldzgradients. Axons are represented using a one‐dimensional compartmental cable model including the kinetics of mammalian myelinated fibers. Computer simulations of the model were performed for sinusoidal and trapezoidal fields. Several axon and field parameters were tested to determine the threshold values for axonal stimulation of the magnetic field, induced electric field and time derivative of the magnetic field. The results indicate that 1) threshold for stimulation is lowest for the largest diameter axons terminating in the region where the magnetic field is maximum, 2) trapezoidal waveforms can be optimized to allow better sub‐threshold resolution than sinusoidal wave‐forms, and 3) the induced electric field is better than the magnetic field and time derivative of the magnetic field as indicators of stimulation thre

ISSN:0740-3194    

DOI:10.1002/mrm.1910380511

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY